Fabrication process for sub-8 nm HfZrO2-based ferroelectric tunnel junctions with enhanced properties

Keeping the miniaturisation pace in the modern semiconductor technology, while chasing an increased computing efficiency, has stimulated the research to focus on novel computing paradigms. One of these is neuromorphic engineering, which aims at the physical implementation of devices mimicking biological neuron and synapses. In this context, memristors based on ferroelectric material are promising candidates to implement synaptic functions. For instance ferroelectric tunnel junction (FTJ) memristors, based on HfZrO2 (HZO) have shown synaptic learning abilities [1]. In addition, HZO processes are already fully compatible with the CMOS industry, with oxide layers thinner than 10 nm. In this work we present a comparative study of different HZO-based FTJs. By interface engineering, we aimed at improving the structural and electrical performances of ultra-thin ferroelectric HZO films. The HZO was synthesized by magnetron sputtering from a Hf0.5Zr0.5O2 ceramic target and subsequently crystallized by rapid thermal annealing [2]. We compared the structural properties and the electrical performances of sub-8 nm HZO layers sandwiched between bottom and top electrodes made of titanium nitride or tungsten. Furthermore, we probed the effect on the ferroelectric properties of HZO of the insertion of an ultra-thin titanium layer at the electrode/HZO interface. The microstructure and the chemical properties of HZO were investigated by means of glancing incidence X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy. Electrical characterization was conducted using the positive-up-negative-down-technique and by the acquisition of current vs voltage characteristics. We propose optimized stacks with enhanced ferroelectricity, which are considered for the implementation of FTJs, and for the demonstration of synaptic learning mechanisms for neuromorphic applications. References: [1] L. Chen et al. Nanoscale, vol. 10, no. 33, pp. 15826–15833, 2018. [2] J. Bouaziz, et al., ACS Applied Electronic Materials 1 (9), 1740-1745, 2019.